The present invention relates to the catalytic hydrogenolysis of Sixe2x80x94X bonds (X=halogen, preferably Cl) present in compounds of the alkylhalosilane (ahs) type, for example methylchlorosilane, for the purpose of converting these ahs into alkylhydrohalosilanes (ahhs), for example methylhydrochlorosilane. In particular, the hydrogenolysis to which the invention relates is of the type that employs gaseous hydrogen and a metallic catalyst with production of hydrogen halide.
More particularly still, the invention relates to the value enhancement of byproducts of the direct synthesis (or Mxc3xcller-Rochow synthesis) permitting the, production of methylchlorosilanes (MCS), which are base monomers for the manufacture of silicones by hydrolysis of Sixe2x80x94Cl bonds and creation, by polycondensation, of polysiloxanes containing siloxyl units xe2x80x9cDxe2x80x9d (xe2x80x94Me2SiO2/2xe2x80x94), xe2x80x9cMxe2x80x9d (xe2x80x94Me3SiOxe2x80x94), xe2x80x9cTxe2x80x9d (xe2x80x94MeSiO3/2xe2x80x94), xe2x80x9cQxe2x80x9d (xe2x80x94SiO4/2xe2x80x94). The direct synthesis takes place by reaction between metallic silicon and methyl chloride at a temperature of between 250 and 300xc2x0 C. in the presence of a catalyst based on copper, zinc or tin. This synthesis leads to a mixture in which dimethylchlorosilane Me2SiCl2 is predominant (approximately 90%), but also to heavier products consisting primarily of disilanes (approximately 8%) of formula MepCl3xe2x88x92pxe2x80x94Sixe2x80x94Sixe2x80x94Cl3xe2x88x92qMeq (p, q=1 or 2). Other, so-called xe2x80x9clightxe2x80x9d MCS are also formed at the outcome of this direct synthesis. These light MCS are produced in allow proportion. They comprise in particular MeSiCl3 (7-18%), MeSiCl2 (0.5%) and, in even smaller quantities, Me2HSiCl, MeHSiCl2, Me4Si. HSiCl3, and isopentane. The light MCS are present in the stream emerging from the head of one or more distillation columns. This distillation operation makes it possible to separate the various products of the direct synthesis.
Among the light MCS, MeSiCl2 constitutes the building block for the formation of polysiloxane chains containing units D (xe2x80x94Me2SiOxe2x80x94); Me3SiCl serves as a chain-end blocker; MeSiCl3 permits the crosslinking of the polymer; and the compounds MeHSiCl2 and Me2HSiCl enable the functionalization of the polymer through the use of the Sixe2x80x94H bond. Me2HSiCl, which does not possess an Sixe2x80x94Cl bond, permits selective chain-end functionalization, which is particularly desired. Consequently, the relative value of the MCS in relation to Me2HSiCl2 (reference=1) is as follows:
Me2HSiCl (10-100) greater than Me3SiCl (2-3) greater than MeHSiCl2 (0.5-1.5))  greater than MeSiCl3 (0.1-0.2).
Industrially, the production of MeSiCl3 far outstrips demand. The desire of those involved in industry would therefore be to enhance the value of MeSiCl3. Since the hydromethylchlorosilanes Me2HSiCl and MeHSiCl2 are greatly desired, it is possible to envisage producing them from the methylchlorosilanes Me2SiCl2 and MeSiCl3, respectively, which are widely available and inexpensive.
Accordingly, a number of processes have been proposed for hydrogenolyzing Sixe2x80x94Cl bonds present in ahs by molecular hydrogen, in accordance with the following reactions:
Me2SiCl2+H2⇄Me2ClSiH+HCl
Me2SiCl3+H2⇄MeCl2SiH+HCl
These reactions are catalyzed by at least one metallic compound.
Among the known processes of hydrogenating alkylhalosilanes, especially methylchlorosilanes, mention may be made of that described in the U.S. Pat. No. 5,329,038, wherein dimethyldichlorbsilane is hydrogenated using hydrogen gas in the presence of aluminum and a catalyst selected from the group consisting of copper, tin, zinc and derivatives of these metals.
The European patent application No. 717 900 describes the vapor-phase catalytic hydrogenation of alkylhalo(chloro)silanes to produce alkylhydrohalo(chloro)silanes in the presence of a metallic catalyst selected from the group consisting of palladium, platinum, ruthenium, optionally supported on active carbon and/or on aluminum oxide and/or on titanium oxide and/or on silicone oxide. More specifically, this European application describes the conversion of Me2SiCl2 into Me2HSiCl, MeHSiCl2 and Me3SiCl. The catalysts employed in the examples are, respectively, palladium on active carbon, platinum on active carbon, and ruthenium on alumina. The gaseous hydrogen is mixed into the Me2SiCl2, which is also in gaseous form. The reaction temperatures are 340 and 400xc2x0 C. under pressures of 2, 6 and 10 bar. It should be noted that, in order to obtain a selectivity for MeHSiCl2 of the order of 50%, it is necessary, according to this process, to employ pressures of 6 and 10 bar, which are relatively difficult to manage at the industrial level.
The European patent application No. 714 901 possesses the same content as the application EP No. 714 900 studied above, except that in this case the hydrogenation catalyst employed is hexachloroplatinic acid.
Within such a state of the art, one of the essential objectives of the present invention is to provide a process for preparing alkylhydrohalosilanes ahhs, in particular monosilanes of the methylhydrochlorosilane kind, by catalytic hydrogenation of alkylhalosilanes (ahs) of the methylchlorosilane type in the presence of a metallic catalyst, the purpose of such a process being to permit the obtention of high selectivities for alkylhydrohalosilanes (especially for methylhydrochlorosilanes) without, moreover, it being necessary to utilize conditions which are drastic and poorly suited to use in industry (high pressure).
Another essential objective of the present invention is to provide a process for catalytic hydrogenation in vapor phase of alkylchlorosilanes, especially methylchlorosilanes, to give alkylhydrohalosilanes, especially methylhydrochlorosilanes, which is simple to carry out and cost effective.
Another essential objective of the invention is to provide a process for enhancing the value of MeSiCl3 by catalytic hydrogenation in vapor phase of this byproduct of the direct synthesis, for the purpose of obtaining MeHSiCl2 which may be usefully exploited in the production of graftable siloxyl units D or else converted into MeHSiCl by chlorine redistribution starting from Me3SiCl (FR No. 96 07 559 and 97 16 047).
Another essential objective of the present invention is to provide a process for preparing alkylhydrohalosilanes (e.g., methylhydrochlorosilanes) by hydrogenating an alkylhalosilane (e.g., methylchlorosilane) using hydrogen gas in the presence of a metallic catalyst featuring particularly high performance and selectivity for MeH and further featuring a low cost price and obtainability on the industrial scale in a homogeneous form.
Another essential object of the invention is to provide a process for preparing ahhs by catalytic hydrogenation of ahs in the presence of a metal catalyst, the purpose of said process being to overcome the disadvantages of the prior art processes.
Given these objectives, among others, the inventors had the merit to select, after long and laborious research and experimentation, a specific ruthenium/tin catalyst which permits all of the abovementioned objectives to be attained, especially as regards selectivity for alkylhydrohalosilanes (MeHSiCl2).
The present invention accordingly provides a process for preparing alkylhydrohalosilanes (ahhs) of formula (I):
R4xe2x88x92mxe2x88x92n SiHmXnxe2x80x83xe2x80x83(I)
in which
R represents independently a C1-C6 alkyl, preferably linear or branched, and more preferably still a methyl,
X represents independently a halogen, preferably chlorine,
m, n=1 or 2 and m+nxe2x89xa63
by catalytic hydrogenation of alkylhalosilanes (ahs) of formula (II):
xe2x80x83xe2x80x94R(4xe2x88x92f) Sifxe2x80x83xe2x80x83(II)
where f=1, 2 or 3in accordance with the reaction:
R(4xe2x88x92p) Si Xp+H2xe2x86x92R4xe2x88x92mxe2x88x92n SiHmXn+Hmxe2x80x2Xnxe2x80x2
where nxe2x80x2+n=p and mxe2x80x2=0 or 1
in the presence of a metallic catalyst, characterized in that the catalyst comprises a bimetallic ruthenium/tin catalytic agent.
One of the fundaments of the present invention is therefore the selection of a particular metallic catalyst which makes it possible to achieve high selectivities for MeHSiCl2 when the hydrogenation starting material consists of MeSiCl3.
The catalytic agents selected in the process of the invention may advantageously be defined through the method of obtaining it. Accordingly, preferentially, said catalytic agent is obtained from the reduction of a ruthenium complex having an electrovalence of xe2x88x924 and a coordination number of 6, the ligands being either a halogen atom or an anion of a tin halide.
More preferably still, the complex corresponds to the following formula (A)
[Ru(SnX3)6xe2x88x92nXn]4xe2x88x92xe2x80x83xe2x80x83(A);
in said formula (A), X represents a halogen atom, preferably an atom of chlorine or bromine, and n is a number from 0 to 2, preferably 1.
In one advantageous embodiment of the process of the invention, the following complexes are employed as catalytic agents:
xe2x80x94[Ru(Sncl3)6]4xe2x88x92xe2x80x83xe2x80x83(A1)
xe2x80x94[Ru(SnCl3)5Cl]4xe2x88x92xe2x80x83xe2x80x83(A2)
xe2x80x94[Ru(SnCl3)4Cl2]4xe2x88x92xe2x80x83xe2x80x83(A3)
The ruthenium and tin halogen complex, selected in accordance with the invention and corresponding preferably to the formulae (A1) to (A3), has the not inconsiderable advantage of being of high quality when obtained as indicated above.
More specifically and more advantageously, this complex may be produced by reacting a ruthenium halide and a tin halide in the presence of an acid.
In practice, and without limitation, the ruthenium halide is a ruthenium(III) halide, in anhydrous or hydrated form, preferably a ruthenium(III) chloride, and the tin(II) halide, in anhydrous or hydrated form, is preferably tin(II) chloride.
For further details regarding the ruthenium/tin catalytic agent used in the process of the invention, reference may be made to the patent application FR 9 513 185, which substantially describes this catalytic agent in terms of obtention and structure.
According to one preferred embodiment of the catalytic hydrogenation process of the invention, a catalyst is used which comprises at least one solid support impregnated with at least one catalytic agent as defined above by the method by which it is obtained. Still in accordance with this preferred embodiment, it is advantageous for the support to be in the form of powder, beads, granules or extrudates, inter alia.
In practice, the support is selected from metal oxides, preferably the oxides of aluminum, of silicon or of zirconium, active carbons, and resins.
With regard to optimizing the process of the invention, it is preferable for the catalyst to have a ruthenium concentration [Ru] defined as follows in % by weight on a dry basis:
0.1xe2x89xa6[Ru]xe2x89xa620,
preferably
0.4xe2x89xa6[Ru]xe2x89xa610,
and more preferably still
1xe2x89xa6[Ru]xe2x89xa68.
In the context of selecting the catalyst according to the invention, it is advantageous for the Sn/Ru molar ratio of the catalyst to be defined as follows:
0.1xe2x89xa6Sn/Ruxe2x89xa630,
preferably
0.4xe2x89xa6Sn/Ruxe2x89xa610,
and more preferably still
1xe2x89xa6Sn/Ruxe2x89xa68.
Regarding the other parameters of employment of the process of the invention, it is useful to specify that the hydrogenation of ahs (II) to ahhs(I) is preferably carried out at a reaction temperature xcex8r defined as follows in xc2x0 C.:
200xe2x89xa6xcex8rxe2x89xa6600,
preferably
400xe2x89xa6xcex8rxe2x89xa6500.
As regards the reaction pressure preferred in accordance with the invention, it is possible to specify that it is in practice less than 2 bar and, preferably, corresponds substantially to atmospheric pressure.
Insofar as the stoichiometry of the reaction at the heart of the process in accordance with the invention is concerned, it is preferred that it is such that the H2/ahs (II) ratio is defined as follows in equivalents:
1xe2x89xa6H2/(II)xe2x89xa6100,
preferably
10xe2x89xa6H2/(II)xe2x89xa620.
According to one preferred embodiment of the invention, the vapor phase hydrogenation of ahs (II) is carried out by heterogeneous catalysis.
It is obvious that, within an industrial system, it is preferable for the process of the invention to be employed continuously, taking measures to ensure that the gas flow rate of the reactants employed, for example MeSiCl3 and hydrogen gas, are such that the contact time Tc of said reactants H2 and ahs (II) with the catalyst is:
between 0.1 and 100 s
and more preferably still between 0.1 and 10 s.
Advantageously, the hydrogenation of (II) to (I) is carried out at atmospheric pressure.
In one advantageous embodiment of the invention, the reactive hydrogen gas employed is mixed with at least one gaseous diluent selected preferably from inert gases, more particular preference being given to nitrogen.
In practice, the diluent gas or gases may represent, for example, from 40 to 60% by volume of the hydrogen gas reagent, preferably approximately 50%.
Regarding the substrate ahs (II) to be hydrogenated, it preferably comprises MeSiCl3, which is converted into MeHSiCl2+HCl.
Owing to the selectivity for MeH which it enables to be attained, the process of the invention offers an advantageous route for enhancing the value of low added value byproducts of the direct synthesis, such as MeSiCl3.
Moreover, this does not exclude the process of the invention possibly being applied to Me2SiCl2, it being understood that in such a case it is evidently not the value enhancement which is intended.
The selection of the appropriate device for implementing the process of the invention is a procedure which is entirely within the scope of the skilled worker.
In any case, the examples which follow will enable better understanding of the invention and perception of all of its advantages and embodiments. Furthermore, these examples provide an illustration of the type of device which may be suitable for implementing the process.